Canadian MineralogistVol. 28, pp.451474 (1990)

GEOCHEMICAL CONSTRAINTS ON THE GENESIS OF THE MONTCALM GABBROICCOMPLEX AND Ni-Cu DEPOSIT, WESTERN ABlTlBl SUBPROVINCE, ONTARIO

C. TUCKER BARRIE* eNo ANTHONY J. NALDRETTDepartment of Geology, University of Toronto, Toronto, Ontario MsS 3Bl

DON W. DAVISJack Sauerley Geochronology Laboratory, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario MSS 2C6

ABSTRAcT

The Montcalm gabbroic complex (X02 + 2 Ma: U-Pbzircon), located in the western Abitibi Subprovince inOntario, contains three lithologic units: a basal pyroxenitezone (PZ), a gabbro - anorthositic gabbro zone (GZ) anda ferroan gabbro zone (FZ). Lower PZ rocks areorthocumulates, with orthopyroxene, clinopyroxene, oli-vine and minor plagioclase as cumulus phases. They haveMg numbers of 68-72, and are enriched in TiO2, P2Os, Zr,Y, and the REE relative to GZ cumulates, reflecting thepresence of abundant interstitial liquid near the margin ofthe complex. They are ZREE-enriched, with La11/Ybp inthe range 2.4-3.8, and are slightly depleted in Ta, Nb, Ti,Zr, and Hf compared to MORB or primitive mantle. UpperPZ rocks have coarse-grained and pegmatitic textures, withcumulus clinopyroxene, orthopyroxene and plagioclase thatare partly or completely replaced by postcumulus horn-blende, plagioclase and Fe-Ti oxides. Pyroxenite andgabbro pegmatite dikes with either sharp or gradational con-tacts are common in the upper PZ. Systematic enrichmentsin V, Cu, Ni, Co, Au and S and, to a lesser extent, the REEare found in upper PZ rocks. They are attributed to con-centration by a zone-refining process, whereby partialmelting ol hot (supersolidus?) pyroxenite was fluxed by avolatile-rich fluid medium, possibly derived from contem-poraneous felsic magmatism (granodiorite dike: 2700i5Ma: U-Pb zircon). GZ rocks are composed of plagioclase- clinopyroxene meso- to adcumulate, with local plagioclasesubporphyritic intervals. GZ whole-rock Mg numbers rangefrom 73 to 81, and have low incompatible trace-element,sulfur and PGE contents, consistent with low amounts oftrapped liquid and sulfur-undersaturated conditions duringGZ formation. The Montcalm deposit (3.56 million ronnes,| .44 wt.t/o Ni and 0.68V0 Cu, 50 ppb total PGE) is locatedwithin GZ cumulates at the northem extent of the com-plex. High Pt,/Ir values and low PG.E contents in the Ni-Cu deposit and lower PZ rocks are consistent with theirderivation from a primitive magma that had experienced1) olivine and chromite fractionation, with Ir behaving com-patibly in the crystallizing phases, accompanied by 2) con-tinual removal of trace amounts of sulfide. The main eventof sulfide segregation may have occurred in a different partof the magma chamber or at depth, in a lower crustalchamber. The deposit was then tectonically emplaced intoits present location from a predominantly pyroxenitic hostduring the late stages of consolidation.

tPresent address: BP Resources Canada Limited, 890 WestPender St., Ste. 700, Vancouver, British Columbia V6C lK5.

Keywords: Montcalm gabbroic complex, Abitibi Sub-province, Ontario, nickel, platinum, magmatic sulfide,zone refining.

SOMMAIRE

Le complexe gabbroique de Montcalm, dans le secteurOuest de la sous-province de l'Abitibi en Ontario, a un dgeU-Pb obtenu sur nrcon de2702 + zMa, et comprend troisunit6s lithologiques, une zone inf6rieure de pyroxenite (PZ),une zone de gabbro et de gabbro anorthositique (GZ), etune zone de gabbro riche en fer (FZ). Les roches pris dela base de la zone PZ sont des orthocumulats d'orthopy-roxdne, de clinopyroxbne et d'olivine, avec plagioclaseaccessoire. Ces roches ont une valeur de Mg#[00Mg/(Mg+Fe)] entre 68 et 72, et sont enrichies enTiO2, P2Oj, Zr,Y et les terres rares compar6es alrx cumu-lats GZ; ces caracteres sont I'expression d'une fractionimportante de liquide interstitiel prbs de la bordure dumassif, Elles sont enrichies en tenes rares l6gbres (2.4< Lap/Yb1 < 3.8) et ldgirement appauvries en Ta, Nb, Ti,Zr et Hf compardes aux basaltes MORB et au manteau pri-mitif. Plus haut dans I'unit6 PZ. les roches ont une tex-ture grossidre a pegmatitique, et sont des cumulats de cli-nopyroxdne, d'orthopyroxdne et de plagioclase qu'ontremplac6 ir un stade postcumulus, complbtement ou enpartie, homblende, plagioclase et oxydes de Fe-Ti. Lesfilons de pyroxenite et de gabbro pegmatitique y sontr€pandus et montrent une 6ponte soit franche, soit floue.Nous signalons des enrichissements syst6matiques en V, Cu,Ni, Co, Au et S, et des terres rares. Nous attribuons cesenrichissements 2r un processus de purification par fusionen zones, c'est-i-dire une fusion partielle d'une pyrox€niteprEs de son solidus par I'addition d'une phase fluide, li6epeut etre i un magmatisme lelsique contemporain (un filonde granodiorite a un dge U-Pb obtenu sur zircon de2700:45 Ma). Les roches de la zone GZ sont des m6socu-mulats et des adcumulats ir plagioclase + clinopyrox0nequi montrent des pass6es localement subporphyriques ir pla-gioclase. Leur valeur de Mg# est situ€e entre 73 et 8l; ellesmontrent de faibles teneurs en 6lements-traces incompati-bles, en soufre et en 6l6ments du groupe du platine (EGP),ce qui concorde avec une faible proportion de liquide inters-ritiel et des conditions de sous-saturatiot-r en soufre. Le gise-ment Ni-Cu de Montcalm (3.56 x 10o tonnes, 1.440/o enpoids de Ni, 0.6890 de Co, 50 ppb des EGP) est situ6 ausein des cumulats GZ dans le secteur Nord du massif. Lesvaleurs 6levdes de Ptllr et les faibles teneurs en EGP duminerai et des roches prbs de la base de l'unit6 PZ seraientl'expression d'une d6rivation d panir d'un magma primitif

451

4s2 THE CANADIAN MINERALOGIST

qui aurait subi un fractionnement de I'olivine et de la chro-mite, le Ir ayant agi de fagon compatible dans cet assem-blage, et d'une perte continue de faibles quantit6s de sul-fures. L'6v6nement principal de s6paration de sulfurespourrait avoir eu lieu ailleurs dans la chambre magmatiqueou a plus grande profondeur, dans la crorlte inf6rieure. Legisement a ensuite 6t6 mis en place tectoniquement par led6placement d'une roche-h6te A caractdre surtout pyrox6-nitique au cours d'un stade tardif de la cristallisation ducomplexe.

(Traduit par la R6daction)

Mots-clds: complexe gabbroique de Montcalm, sous-province de I'Abitibi, Ontario, nickel, platine. sulfuremagmatique, affinage en zones.

INTRODucrroN

The Montcalm Ni-Cu deposit was discovered in1976. when the first drillhole to test an airborne elec-tromagnetic anomaly intersected 14.4 m of massiveand disseminated sulfide mineralization (Blecha etal. 1977). Further drilling outlined 3.56 milliontonnes of reserves to a depth of 300 m, with an aver-age grade of l.44Vo Ni and 0.6890 Cu. The deposit

is unusual in two respects. First, the sulfide miner-alization is hosted within cumulate gabbroic rocks.Generally, Ni-Cu sulfide mineralization is found atthe base of the host intrusion, a feature commonlyattributed to gravity settling of a dense sulfide liquidthrough a less dense silicate liquid. Barrie & Naldrett(1989) interpreted the deposit as having been tecton-ically emplaced into its present location from apredominantly pyroxenitic host, during or shortly af-ter consolidation of the Montcalm Gabbroic Com-plex (MGC). The second unusual feature of thedeposit is its very low PG,E tenor (average of 50 ppbtotal PGE), despite Ni and Cu contents that are con-sidered normal for gabbro-hosted magmatic sulfidedeposits (e.g., nearly two orders of magnitude low-er than typical Sudbury deposits: Naldrett l98l). Aninitial explanation for the low tenor of the PGE atMontcalm was provided by Naldrett & Duke (1980).They modeled the average Ni, Cu and Pd contentsof the deposit as a product of a sudden event of sul-fide segregation from a picritic magma, after themagma had undergone continuous segregation oftrace amounts of sulfide during the fractionation ofolivine, clinopyroxene and plagioclase.

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Ftc. l. Location map, and regional geology in the western Abitibi Subprovince. The contact with the Kapuskasing Struc-tural Zone represents a west-dipping thrust fault.

IHE MONTCALM GABBROIC COMPLEX. ONTARIO 453

The geology and strucrural history of the MGCand Ni-Cu deposit have been described in a com-plementary study (Barrie & Naldrett 1989). Thepresent study uses a detailed petrographic, mineraland whole-rock geochemical traverse across thestratigraphy to interpret the magmatic and postmag-matic processes that formed the MGC and Ni-Cudeposit. High-precision U-Pb zircon ages arepresented for the MGC and a granodiorite dike thatcuts the Ni-Cu deposit, to place these rocks withinthe temporal framework of magmatism and tec-tonism in the southern Superior Province. In addi-tion, neodymium isotope analyses from a regionalstudy (Banie & Shirey, submitted) are used to charac-terize isotopic signatures of the MGC parental

magma and evaluate the extent of assimilation thatmust have occurred during the formation of the gab-broic complex.

REcToNAL Geot-ocrcet- SErrrNc

The Abitibi Subprovince, a major lithologic - tec-tonic block of the southern Superior Province, ischaracterized by predominantly east-trending low-grade metavolcanic and metasedimentary rocks, andmassive to gneissic intermediate and felsic plutons.The Kapuskasing Structural Zone is a northeast-trending elongate belt (500 x 59 km) ofhigh-grademetamorphic rocks at the western terminus of theAbitibi Subprovince. The Montcalm area comprises

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Frc. 2. Generalized geology of the Montcalm area.

454 THE CANADIAN MINERALOGIST

a discrete assemblage of supracrustal volcanic,sedimentary and intrusive rocks, and is located alongthe eastern border of the Kapuskasing StructuralZone at the extreme western margin of the AbitibiSubprovince (Fig. l).

Geological studies in the Montcalm area have beenlimited owing to poor access and exposure. Previousmapping by Bennett (1966a, b, 1969) has describedthe general geology to the south and central partsof the Montcalm area. The MGC is centrally locatedin the Montcalm area, within iron-rich tholeiiticbasalts, and calc-alkaline andesites and dacites. Con-tacts with the surrounding rocks are rarely exposed.In drill core, metavolcanic rock - MGC contacts areinterdigitaring and gradational, indicating that partsof the MGC extend laterally into the metavolcanicsuite.

Mafic metavolcanic rocks predominate to thesouthwest and are locally interbedded with fine-grained metasedimentary rocks. Shallow northwest-dipping foliations and attitudes of bedding suggestthe structural influence of the Kapuskasing thrustfault to the northwest. The mafic volcanic rocks aremetamorphosed to amphibolite grade within 3 kmof the contact with Kapuskasing gneissic felsic intru-sive rocks. The northern third of the Montcalm areaconsists of high-iron tholeiitic mafic metavolcanic

rocks interbedded with fragmental and tuffaceouscalc-alkaline felsic metavolcanic rocks. Diamonddrilling northwest of the MGC encourtered gabbroicsills up to 40 m thick within the metavolcanicsequence. In addition, two sulfidic iron formationshave been intersected by drilling along strong linearairborne magnetic anomalies (Fig. 2). To the east,the MGC is bordered by a large composite tonalite- granodiorite - granite batholithic complex. Asmaller gneissic tonalite cuts the MGC along itssouthern margin, and ultramafic sills and dikes(clearly detected by air magnetic surveys: Fig. 2) cutthe intrusion and neighboring metavolcanic rocks.

GEoLoGy oF THE MoNrce.lu Ga.ssnoIc CoMPLEX

The MGC is a large (85 km2) subvertical, crudelylayered crescent-shaped intrusion (Fig. 2). Detailedintrusion-wide lithological or structural studies arenot possible, as exposures are limited to those alonga river in the southern part of the intrusion, and toscattered outcrops within the intrusion. However,based on these limited exposures, the MGC can bedivided into three zones: a Pyroxenite Zone (PZ),Gabbro - Anorthositic Gabbro Zone (GZ) and Fer-roan Gabbro (FZ), which broadly define a NNW-SSE Fe-enrichment trend (Figs. 3,4). Four dike suitescut the MGC, including peridotitic, pyroxenitic, gab-broic and granitic dikes. All rocks have been sub-jected to varying degrees of subsolidus alteration.Clinopyroxene is commonly partly or completelyreplaced by tremolite-actinolite and hornblende, andplagioclase is commonly altered to sericite, chloriteand clay. The following descriptions emphasize theprimary cumulus and postcumulus mineralogy deter-mined from outcrop, hand specimen and petro-graphic observations, and microprobe analyses oftheleast-altered samples.

Rock Types

The Pyroxenite Zone occurs 300 m northwest ofrhe Ni-Cu deposit, as a lens-shaped mass up to 250m wide and 450 m long (Fig. a). Pyroxenitic rockscomprise 4090 of the PZ. They are massive, medium-grained clinopyroxene orthocumulates, with cumulusolivine and orthopyroxene, and rare plagioclase.Intercumulus silicates include orthopyroxene,plagioclase and oikocrystic hornblende. Pegmatiticgabbroic rocks comprise 3590 of the PZ, and aremore prominent near the PZ-GZ contact. They havesubequal proportions of primary plagioclase,clinopyroxene and hornblende, with up to 590magnetite and ilmenite; chlorite is commonly pseu-domorphous after the silicate phases. Both of theserock types have up to 4Vo sulfides, predominantlypyrite and pyrrhotite, with lesser chalcopyrite andpentlandite locally. The remainder of the PZ con-

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THE MONTCALM GABBROIC COMPLEX. ONTARIO

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Frc. 4. Rocks of the Montcalm Ni-Cu deposit area, with Mg numbers for representative drill-core samples (projectedto surface). The Ni-Cu deposit is divided into West and East Zones (hachured) by a subvertical granodiorite dike(cross pattern); a more siliceous dike borders the West Zone on its western margin (cross pattern). Line A-A' usedfor geochemical traverse in Figure 6.

455

4 3 4. -

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sisrs of pyroxenitic and gabbro pegmatite dikes thathave both sharp and gradational contacts with sur-rounding rocks. P,.roxenitic xenoliths are presentwithin the Ni-Cu deposit, and within granitic rocksmarginal to the MGC.

The Gabbro - Anorthositic Gabbro Zone is greaterthan 800 m thick and primarily composed of mas-sive medium- to coarse-grained clinopyroxene-plagioclase mesocumulate and adcumulate. The GZhosts the Ni-Cu deposit (Fig. 4). Poorly developedmodal layering is present in approximately 590 ofthe drill core. Modal abundances of plagioclase andclinopyroxene vary from 30 to 6590, and inter-cumulus phases are generally less than 1590. Anor-thositic gabbroic rocks are more abundant east ofthe Ni-Cu deposit. They are coarse-grainedmesocumulates with greater than 6590 plagioclase,and with plagioclase and clinopyroxene as bothcumulus and intercumulus phases. Modal gradationsare visible on a scale of meters in approximately l5Voof the drill core. In comparison to the PZ, the GZhas a higher proportion ofplagioclase, rare cumulusorthopyroxene, fewer intercumulus phases and isgenerally less altered. The contact with the PZ ismarked by the appearance of plagioclase as a

cumulus phase, and is gradational on a scale ofmeters. Gabbro pegmatite dikes are rare inthe GZ,and are present only west of the Ni-Cu deposit.

The Ferroan Gabbro Zone is located in thesouthern two-thirds and the western "tail" of theMGC (Fig. 3). It is characterized by the presence ofinterstitial and cumulus magnetite, along withmedium- to coarse-grained plagioclase, clino-pyroxene and hornblende. Several of the outcropshaveFZ dikes, or crystal mushJike FZ material thatcut FZ cumulate rocks. Xenoliths of PZ cumulates arefound within FZ dikes, locally. Primary modallayering is present locally, as mafic to felsic gradedlayers up to I m thick and as centimeter-thick wispylaminations of mafic minerals within the moreplagioclase-rich layers.

Ultramafic, mafic, felsic and alkaline dike suitescomprise fully 2590 of the drill core in the vicinityof the Montcalm deposit. The ultramafic and maficdikes may represent crystal mushes of MGC materialthat were mobilized during tectonism and consolidation of the cumulate rocks. A geochemical surveyof these dikes indicates that in general, they do notrepresent compositions of the parental liquid. Thegeology, petrology and geochemistry of the dike

456 THE CANADIAN MINERALOGIST

TABLT 1. U.Pb ISOTOPI DATA FOR ZINCONS FROM NO]\] 'TCAIM GAEBROIC COMPI.TX A(D GNAIODIORITT DIKE SMPLTS

FRACIToNSa WEICHT U COi0,rOli Pb! z!f!!c A!l!!0 ?!-9!U0 .?_0-6-L!e llfgUe I 207Pbs porcrnt

( n g ) ( p p m ) ( p S ) 2 0 6 P b 2 0 1 P b 2 0 6 P b 2 3 8 U 2 3 5 U 2 0 6 P b d l s c o r d l n l(Ase)

MGC-l foNrcAtlr GABBRoTC Co*iPL[X

1 . 0 N , { 2 0 0 A b 0 . 0 1 2 1 6 3 8 0 0 . 1 9 8 3 8 1 5 5 0 . 1 7 8 0 . 5 1 9 2 1 1 3 . 2 6 1 2 1 0 1 0 . 2 4

3 8 r a i n s 1 8 5 ( 0 . 6 0 ) ( 0 . 1 1 )

2 . 0 N . + 2 0 0 A b 0 . 0 2 0 9 2 3 0 0 . 1 E 9 0 5 3 6 7 0 . 2 3 9 0 . 5 1 6 7 0 1 3 . 1 9 1 2 6 9 9 . 1 0 . 6 4

3 8 r 8 i n s 5 E 9 ( 1 . 2 ) ( 0 . l 2 )

3 . 0 N , + 2 0 0 A b 0 . l l 5 2 0 9 1 5 0 . 1 E 5 0 8 3 3 9 3 0 . 1 8 5 0 . 5 1 6 ! 7 1 3 . 1 1 2 2 6 9 8 0 . 6 3

4 oN, +200 o 01s 266 8 0.18474 , ,uu o.zo{ 0 51121 l i i l l l : r : t ' r 513 B r a i n r ' ( 0 . 8 0 ) ( 0 . l 0 )

86-220 [ !0iJTCALU GRAI' ]oDt0RlTE DtXt5 . o i l , + 2 0 0 A b 0 . 0 1 9 8 5 I 0 . 1 E 1 9 2 6 7 9 0 . 0 2 0 0 . 5 1 7 { 5 1 3 . 1 9 0 2 6 9 7 0 . 3 9

5 8 r l l n s . ( 0 . 8 0 ) ( 0 . 2 0 )

6 . 0 N . + 2 0 0 A ! 0 . 0 0 9 2 0 9 1 1 0 . 1 8 5 8 0 1 9 1 0 . 1 3 2 0 . 5 1 3 1 3 1 3 . 1 1 4 2 7 0 0 . 3 l . 3 l

5 g r r l n s 4 1 t ! ( 1 . 0 ) ( 0 . 1 0 )

7 . 0 l ' / , + 2 0 0 0 . 0 1 2 5 1 5 1 0 0 0 . 1 E 7 7 5 3 4 1 0 . 1 2 3 0 . 1 7 1 3 5 1 1 . 8 1 3 2 6 6 9 6 . I

3 s r ! I n s 3 E 3 ( 0 . 8 0 ) ( 0 . 1 0 )

s ) A l l l r a r t l 0 n s a r e z r r c o n p o p ! l a t i o n s ! n l o s s o t h 0 f i i s 6 I n d i c a t o d . o N r O 0 l i l t , n o n - n a g n e t l c l r s c t l o n t r o n

n a g n e t l c s r p s r a t o r ; A b : ! i r . r b r s d o d l r a c t l 0 n ( ( f o g h l 9 E 2 ) ; + 2 0 0 / - 2 0 0 : f r a c l l o n i l a f g s r 0 . s n a l l o r t h a n 2 0 0

s i z o n s s h ( 8 b o u t 7 1 l n ) . r o l p s c t l y e l t .

b ) C 0 f r e t l e d f o . 0 . l l 7 n o l e l r a c t i o n c o m o n P b i n t h o 2 0 5 P b s p l t o . l n c l u d o s b l a n k p l ! s z i r . o n t o m o n P h .

c ) C o r r e c t ! d l o r l r a c t l o n a t i o n r n d b l s n k . P b a n d U i r a r i l o n a t l o n c o r r 6 c t i o n - 0 . l 3 $ p e r a t o n i c n S s s u n i t

u b l a i t = 3 . 8 p g ( l 8 r g B c o l u n n ) , 0 . 5 p g ( s m ! l l c o l u n n , u s o d l o . s s n p l o n u h b o r s 1 , 2 , 6 ) ; P b b l , n k ( 8 . 2 p B

( b o t h c o l u n n ! ) .

d ) ! l e a s ! r , d t a l u g s . I u h b e r s i n b r a c k s t s c 0 r r o a t B d l o r l r s a t i 0 n a t i o n , b l a n k o n d c o m o n l B a d l o t h e s p i k 0 .( ' ) n ! , n ! a l I c o m o n P b c a n b s c o n s i d 0 r o d b l a n k .

e ) C o r r r c t s d l o r l r a c t i o n a t i o n , b l a n k a n d c o m o n P b u s l n g t h 0 m o d s l o l S t a c s y & l ( r s n o r s ( 1 9 7 5 )

t ) i l u n b e r s l n p s r o n t h o s o ! r o l 0 r t 0 p 6 r c e n t 2 - s l g n a o r r o r i n P b / U .

B ) [ ! n b o r s i n p a r e n t h 0 s e s r o { s r t o p s f c s n t 2 . s i g m a o r r o r 1 1 2 0 7 p 6 7 2 0 6 p 6 A n e r r o r 0 f 0 . l s c 0 r r r s p o n d j

t o 1 . 6 l i a .

suites have been described by Barrie & Naldrett tectonic intrusive bodies, which have U-Pb zircon(1989) and Barrie (1990). ages between 2702 and 2668 Ma (Barrie & Davis

1990, Corfu et al. 1989, Davis el al. 1989, andU-Pb zircon geochronologt and structural history references therein).

The MGC has been subject to at least two defor-Two samples were selected from the Montcalm mation events. The first produced a regional penetra-

area to establish the timing of tholeiitic intrusive tive fabric and may have been responsible for bulkactivity and deformation, and for comparison with crustal rotation of the regional stratigraphy into itstholeiitic magmatism and deformation in the Abitibi present subvertical position. The arcuate shape ofand Wawa Subprovinces. A pegmatitic gabbroic theMGCisparalleltotheregionalpenetrativefabric,segregation containing 20/o quara was selected from and suggests that a broad doming to the northwestthe GZ, well away from the upper PZ gabbro peg- may have been coincident with or responsible for thismatite dikes, and a second sample was collected from first event. Along with four outcrops in the FZ thata l5-m wide granodiorite dike that cuts the Mont- show facing directions from modal layering (Fig. 3),calm Ni-Cu deposit. The U-Pb data (Table l) give this indicates a general facing direction to the eastages with overlapping 2o errors of 2702 t 2 Ma for and southeast. The second deformation event, prob-the MGC, based on a four-point discordia line, and ably related to the emplacement of the granitoid2700 x. 2 Ma for the granodiorite dike, based on rocks to the east (2696 - 2694 Ma:. Barrie & Davisa three-point discordia Iine (Fig. 5). Neither sample 1990) and south, produced a strong flattening andshows signs of inheritance. The age for the MGC is elongation fabric on the eastern and southern partsyounger than the U-Pb zircon age for the Kamiskotia of the MGC (Barrie & Naldrett 1989).gabbroic complex 25 km to the ESE (Fig. 1), whichis 2707 + 2 Ma (Barrie & Davis 1990). The MGC GEocHEMISTRyis among the youngest tholeiitic intrusions knownin the southern Superior Province, which were Samples representing the cumulate stratigraphyemplaced between 2750 and2692Ma. The granodi- were taken from several drill holes across strike andorite dike is among the oldest of the syn- to post- perpendicular to the PZ - GZ contact for a petro-

2702+l-2

THE MONTCALM GABBROIC COMPLEX, ONTARIO 457

0 . 4 8 L

12.4 12.8 15.2 13.6207Pbt235U

graphic, mineral, and whole-rock geochemicaltraverse across the Ni-Cu deposit (Fig. 6). Thewesternmost PZ, which represents the floor or thewall of the MGC, is at the base of the traverse.Results of major, trace, and rare-earth-element ana-lyses for 20 representative samples, including 15 fromthe traverse, one pyroxenite xenolith and four fromoutcrop are included in Table 2. Concentrations ofthe platinum-group elements are included for 12 ofthese samples also. All samples weighed 1.5-2.5 kg.Drill-core samples represent 0.5-3.0 m of the section,with the coarse-grained rocks represented by longersections wherever possible. All sample locations andadditional geochemical data for the other samplesin Figure 6 are reported in Barrie (1990).

Petrography and mineral chemistry of rocks on thegeochemical traverse

The mode and grain size of cumulus and inter-cumulus silicate phases are plotted versus strali'graphic height in Figures 7 and 8, respectively. PZsamples are divided on the basis of textural relation-ships into lower and upper groups, below and above100 m in stratigraphic height. Lower PZ rocks haveorthocumulate textures that are easily discernable inhand specimen and thin section. Cumulus phases areprincipally clinopyroxene and orthopyroxene, withsubordinate olivine and rare plagioclase. Inter-cumulus phases include both pyroxenes, plagioclaseand minor Fe-Ti oxides. The unusually large grain-size of the intercumulus phases in sample 86-394 isdue to oikocrystic hornblende. For upper PZ rocks,

0.54

t2.O 12.4 12.9 '13.2 13.6

207Pb1235u

textural relationships that normally distinguishbetween cumulus and intercumulus phases becomeobscure with increasing grain-size. Most of thesesamples have large, optically continuous pyroxenegrains along with oikocrystic hornblende; in addi-iion. interstitial Fe-Ti oxides and sulfides compriseup to 590 of the mode. Textures indicating replace-ment of cumulus pyroxene by hornblende are evi-dent in samples 86-399 and 86-391. Sample 86-403is exclusively pegmatitic gabbro, with completereplacement of cumulus phases by pegmatitic horn-blende, clinopyroxene and plagioclase. In the GZ,clinopyroxene and plagioclase are the cumulusphases; cumulus olivine is present in a sample at308 m height (sample 86-428: Barrie 1990). Petro-graphic estimates of the mode of intercumulus phasesare:25-45t/o in the lower PZ, l0-30t/o in the upperPZ and <3-200/o for the GZ. These proportions arelower than expected considering the levels of incom-patible trace elements (discussed below).

The An content of cumulus plagioclase and theMg number of cumulus clinopyroxene (microprobedata) are plotted along with normative An andwhole-rock Mg numbers, corrected for their sulfidecontent (Fig. 9). Measured plagioclase An contentsare variable but generally lower than the normativeAn values. ln thePZ, the presence of intercumulushornblende in several samples is not included in thenormative calculation, and this tends to increase thecalculated An content. Thus the trend toward nor-mative An values of 90-100 near the top of the PZis partly artificial. In the GZ, one sample has twocore-rim pairs that indicate normal zonation, sug-

Frc. 5. Concordia diagrams for the Montcalm gabbroic complex and granodiorite dike. Ages and 2o errors calculated

using the regressio:n program of Davis (1932). a. Montcalm gabbroic complex. b. Granodiorite dike.

458

wTHE CANADIAN MINERALOGIST

gesting re-equilibration with a more sodic intersti-tial liquid.

A comparison of clinopyroxene and whole-rockMg numbers in the GZ reveals two importanl fea-tures. First, with one exception the most magnesianclinopyroxene grains have Mg numbers from 78 to82, higher than the whole-rock values. Assuming thatthese represent primary, undisturbed cumulusclinopyroxene grains, their consistently high Mgnumber suggests a derivation from a parental liquidthat did not undergo significant closed-system frac-t ionat ion and Fe-enr ichment . Second, re-equilibration with an Fe-enriched, postcumulus silicate liquid may have affected many of the cumulusclinopyroxene grains in the GZ and the lower pZ.This is indicated by the wide range in clinopyroxeneMg numbers, with a maximum range within a sampleat 203 m height (sample 86-402: Barrie 1990)from 82 to 68, and clinopyroxene core-rim pairs withconsistent, normal zonation to a more hedenbergiticrim. Barnes (1986) calculated the "trapped liquidshift" for mafic minerals in the presence of variableamounts of fractionating intercumulus liquid forclosed systems. He found a significant shift towardmore Fe-rich compositions during postcumulus frac-tionation of interstitial liquid, particularly fororthocumulates with a small proportion of maficphases. For example, a gabbronorite with a 60:4eratio of cumulus plagioclase to pyroxene, and with

'""13"-n u

ta6-lzt

50Vo initial interstitial liquid, will see pyroxene Mgnumbers decrease from 80 to 64 (Fig. I in Barnes1986). This example may pertain to many of the GZcumulates, which implies a relatively high propor-tion of original "trapped" or intercumulus liquid inthe GZ. A similar conclusion was reached by Nal-dretl et al. (1970) for the basal norites of the Sud-bury Irruptive, where most orthopyroxene cores havea constant Fe/(Fe+Mg) atomic ratio, yet the vastmajority show a trapped liquid shift of 5-18 atomicolo .

Interestingly, an adcumulate pyroxenite xenolith(sample 86-FRAG: Table 2) sampled near the Ni-Cu deposit does not show any significant zonationin clinopyroxene; nine analyses, including 3 of core-rim pairs, show.M/g numbers from 75 to 80 (Barrie1990). These values are similar to the whole-rockuncorrected Mg number of 75, or 77 if the Fe con-tent is corrected for a petrographic estimate of itssulfide content. It would appear that this pyroxenitexenolith was derived from a part of the MGCchamber (or a separate chamber in the lower crust)that was not affected sienificantly by an Fe-rich post-cumulus silicate liquid, in contrast to the lower PZcumulates.

Postcumulus Fe enrichment in the pegmatiticupper PZ rocks appears to be related to volatileactivity. Intercumulus Fe-Ti oxides comprise 1-4Voof the mode in hornblende-bearing samples.

3 . o

Ftc. 6. Cross section for geochemical traverse, with traverse line (1 15"N, 30.E) and sample locations projected intoplane of section. Labeled samples included in Table 2; geochemistry for other iamples given in Barrie (t99b). Sarnplestratigraphic height along traverse line has error estimated at + 10 m. Location of traverse line in Figure 4.

THE MONTCALM CABBROIC COMPLEX, ONTARIO

!M 2- (m G @ffi ffi OF m UK rc m N m rcW GeSrcrC m

459

8 6 - 4 2 7 8 6 - 4 0 9 a 6 - F S C

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t o r A L 9 8 . 3 9 9 . 0 9 9 . 2 9 6 . 2 9 9 . 1 9 9 . 1 9 7 . 5 g g . 6 9 9 . 5 1 0 0 . 4 g a . 7 9 8 . S 9 1 , 3 9 7 . 8 9 8 ' 3 9 9 ' 2 9 9 ' 9

2 , 1 34 . 62 . 2

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t 6 . j 6 . j 5 . 4 5 . 7 6 . 1 6 . s l o . o 1 o : 9 1 3 : 9o - 2 3 O . 1 l O . 1 l O . 1 0 0 . 1 2 O l z 0 1 3 o t 7 o ' 2 2

2 O . g r r . 1 8 . 5 8 . 1 1 1 . 8 1 2 . 4 1 2 . O 1 6 : ? ! 7 : ?6 . 3 0 9 . r 7 1 1 . s s 1 1 , 7 0 9 . 0 8 s . 1 1 6 - 5 4 6 . ? 9 ' . 3 - 5 -0 . 1 8 1 . 3 3 1 . 4 8 1 . 5 6 1 . O 1 1 . 0 9 1 . e 9 1 ' 1 1 9 ' . : lo . 0 4 0 . 1 7 0 . 3 4 0 . 5 3 0 . 5 2 0 . 1 8 0 . ! ? 9 . 9 9 9 . 9 90 . 0 4 o . 0 3 o - o 2 o . 0 4 o . 0 2 o . o 1 0 . 9 ? q ' 9 1 9 ' 9 1a . o a 2 , 7 1 2 . g 8 4 - 5 4 4 . 0 8 2 . t 5 5 , 2 8 4 ' 2 3 4 ' 3 9

4 0 . 9 4 5 . 6 4 1 . 2 4 7 . 1 3 8 . 40 . 5 6 0 . ? 0 0 . 3 4 0 . 5 2 0 . 3 41 0 . 5 8 . 0 1 0 . 8 1 0 , 1 8 . 31 5 . 7 1 4 - 3 1 1 . 3 1 3 , 6 1 5 . 30 . 2 2 0 . 2 3 0 . I 8 0 . t 1 0 . 2 01 8 . 3 1 3 . 6 1 7 . 2 1 3 . 1 1 9 . 97 . l 3 9 . 4 2 6 . 5 7 a . 6 7 6 . 8 70 . 5 0 1 . 3 9 0 . 4 4 1 . 0 8 0 . 0 40 . 1 3 a . 2 1 0 . 0 9 0 . 2 3 0 . 0 30 . 0 9 0 . 1 0 0 . 0 3 0 . 0 5 0 . 0 55 . 1 6 2 . 6 2 4 . 9 3 3 . 8 5 8 . 2 1

1 . 5 9 1 . 3 9 2 . 1 6 1 . 5 03 . 2 2 . 6 5 . 5 4 . 4X , 6 1 . 7 3 . 5 1 . 8

0 . 3 0 0 . 7 4 0 . 4 7; . i ; o . r e o . 2 o . 2 4

o . 2 . . .o . : i o . 2 B 0 . 3 8 0 . 3 10 . O S 0 . 0 5 0 . 0 8 0 . 0 5

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8 1 . 88 3 0 < I1 4 6 0 - l

< 1 < t0 . 1 < 0 . 0 2

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2 t . 4 t 1 . 5 3 7 . 8 1 6 . 2 3 7 . 3 3 0 . 81 0 4 1 0 8 1 4 0 1 5 9 2 0 0 3 5 61 0 0 5 2 1 0 1 2 1 2 4 6 r 7 3

2 6 < 1 1 7 < 1 1 5 1 16 2 2 3 a 2 4 8 0 1 1 6 4 4 6 3 3 28 1 6 7 5 2 3 5 5 2 0 8 8 1 6 2 9 5 9 3

1 1 3 . 3 X 0 3 . 0 0 1 . 0 1 ? 9 . 5 1 1 1 . 6 9 2 . O13 13 81 2450 2720 291

1 3 0 1 4 1 1 0 9 8 5 1 1 05 2 0 9 8 0 4 8 0 6 5 0 0 I 1 6 0 3 2 4 0

1 t 1 3 7 2 6 A 1 1 1 1 1 a

3 4 0 4 1 0 5 1 0 4 3 0\ 9 6 1 3 4 1 4 2 X 8 6

4 3 . 8 4 4 . 8 8 1 0 4 6 . 22 < 1 < I < l

0 . 1 0 . 1 0 . 9 0 . 16 6 4 4 5I 2 3 1 3 1 l

r . 1 2 . 1 9 . 3 I . 63 1 1 1 0 3 6 2 7 5 2 1

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0 . 9 9 0 . 9 4

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8 84 5

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lI20 720

1 2 7 4

2 . 4 3 . 3 2 2 . 95 . 4 8 . 0 6 . 92 . O 3 . 4 3 . 4

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0 . 1 o . 20 . 2 4 0 . 2 8 0 , 3 20 . 0 5 0 . 0 4 0 . 0 6

2 9 1 5 1 43 4 3

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234 492 4554 1 2 2 0

932 6t1 5021 4A 270 214

4 0 . 3 3 5 . 0 2 A , 72 2 3 1 5 45 0 1 8 . 8 1 1

1 0 0 4 6 0 2 4 0

t 0 1 0 1 7 1 3

2 0 . 3 l 7 , 4 2 5 . 2 2 3 . I6 9 7 3 9 1 8 6

2 O O 2 1 2 5 2 5 51 8 < 1 , . .

9 0 3 5 7 5 ! 1 4 6 8 6 64 3 j 3 9 6 6 1 5 2 3 0 0

5 2 . 2 4 3 . 2 5 9 . 7 1 0 1 . 05 4 9 2 3 62 1 3 8 1 5 06 0 7 2 0 < 2 O

g o 1 3 1 8 ( 7 5 )

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3 . 5 v . t9 1 < 2 3

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s6 ppbAes bO BI !RuPtAu (NS)Au ( lNs)

Microprobe analyses of the Fe-Ti oxides indicate thatthe majority are either magnetite, or ilmenite con-taining from 2.7 lo 4.6 wt.ulo CrrO, with negligibleMg contents (Barrie 1990). The composition of thechromian ilmenite is distinct from that of kimber-litic ilmenite, which contains >2 wt.9o MgO(Mitchell 1986). In many respects, its occurrence issimilar to that of unusual titanium - chromiumspinels that occur in the transgressive, metasomaticultramafic magnetite pipes of the eastern BushveldComplex (Cameron & Desborough l9U, Cameron& Glover 1973, Viljoen & Scoon 1985); however,chromian ilmenite has not been documented in theBushveld occurrences.

Major, trace and rare-eorth-element Seochemistry

For the cumulate rocks of the lower PZ a\d GZ,the majority of the major and incompatible trace ele-ments plot as coherent trends on interelement vari-ation diagrams, and are believed to represent the pre-alteration compositions despite subsolidus alterationand metamorphism. This immobile behavior is inaccord with studies on the mobility of major andtrace elements in volcanic rocks that have been sub-

jected to greenschist-facies metamorphism andnormal processes of surficial weathering (Menzies e/ol.1979, Ludden et al.1982, Beswick 1982). Varia-tion diagrams show that the alkalis and Se, As' andSb do not correlate well with elements of similargeochemical affinities ; therefore' little emphasis -isplaced on these elements. Postcumulus rare-earth-ilement (R,EE) redistribution is evident in thepredominantly pegmatitic upper PZ rocks; this is dis-cussed below. Subsolidus chloritization has resultedin magnesium addition and silica loss in the majorityof PZ rocks, and subsolidus sulfur loss is noted forseveral samples with concentrations of <250 ppm S'

Selected major- and trace-element contents andtrace-element ratios are plotted versas stratigraphicheight in Figures l0 and 1l' The division betweenth;PZ and the GZ is clearly evident in the major-element variations, and reflects the addition ofplagioclase as a cumulus phase in theGZ. The majorile-"ntt remain relatively constant throughout theGZ, except from 350 to 380 m, where MgO andFe2O3 increase at the expense of Alpr, reflegline aninireise in modal clinopyroxene (Fig. 7). For thelower PZ, where primary cumulate texturespredominate, the corrected Mg numbers are rela-

1 3 8

8 0

< 2 0< l

: i< 0 . 0 2

< 2< 1

0 . 42

325

1 6

< 2 0< 1

: i0 . 0 5

< 2< l

7 . 22

3 5 01 5 1 2 4 2

t ra jo l € febents by xRF a t x -Ray Assay Labora to l ies , to lon to ; t rac€ € lenents by xRF and w 6 t !h6 uo lve tB i ty o f ?oron to ;

PGE bv M a t lbe u l l vers r ty o i ro ron to ; see Ba l r ie (1990) fo r expLana l ion o f ana ly t i ca l techn iquea and the l r accuracy

; i l ; ; . ; - i ; j " ; . - i s , i ,

us nun ie r ca tcu ta ted us lng no t€ percent Mgo/ uso { Feo, w i th Fe in F6o = 0 '85 to ta f Fe , f€ss Fe in

nonosu l f ide so t id so tu l ton es t lna ted us ing FeTS; . fo ieaupfe A i - rns , tne ug nudb€r i s no t co t tec tea l f ,o l au l f , lde conten t

460 THE CANADIAN MINERALOGIST

500

ro[!r

xotrE 200

$ aoo

=amNmEol opx cpx hb plag S*mt

FIc. 7. Mode (cumulus + intercumulus) for geochemicaltraverse samples, based on petrographic estimations.Height refers to west - east distance along cross-sectionline in Figure 6. Symbols: ol olivine, opx orthopyroxene,cpx clinopyroxene, hb hornblende, plag plagioclase,S+mt sulfides + Fe-Ti oxides.

tively low, from 68 to 73. Mg numbers graduallyincrease up-section across the PZ-GZ contact, withvalues from 70 to 75 in the upper PZ steadilyincreasing up to 79 to 81 in the middle GZ.

The compatible trace-elements Cr, Sc and V par-tition into the mafic silicate and oxide phases. Whole-rock Cr contents are generally lower in the pZ thanin the GZ, where trace amounts of chromite arepreserved locally. Chromium distribution is erraticbut broadly follows the corrected Mg number, sug-gesting an overall trend toward more primitive com-positions up-section. Scandium contents generallyfollow the modal abundance of clinopyroxene in theGZ. ln the lower PZ, samples l-3 have low Sc con-tents (17.5 to 21.7 ppm) that reflect the higher modalproportion of olivine and orthoplroxene, which havepartition coefficients well below unity for Sc (Lind-strom 1976).

Vanadium appears to be a particularly sensitiveindicator of volatile-related, postcumulus enrichmentprocesses in the upper PZ. Strong enrichment in Voccurs from 100 to 140 m, where, as previously men-

I

*

grain size (mm)cumulus intercumulusphases phases

Frc. 8. Cumulus and intercumulus grain-size ve,'sas strati-graphic height for geochemical traverse samples, basedon petrographic estimations. PZ divided into lowerorthocumulates (unshaded), and upper (shaded) rocks,which are predominantly subpegmatitic to pegmatitic.

tioned, the cumulate textures are obscured by par-tial to complete replacement. by pegmatitic phases.These samples contain 1-3Vo interstitial Fe-Tioxides. The mineral-liquid partition coefficients forV between minerals and mafic to intermediate sili-cate melts are generally greater than unity, but theyare strongly dependent on the oxygen fugacity of thesystem, with higher partition coefficients for morereduced systems. They range from 5 at anfiO) of10-12 bars to 0.05 at anf(Or) of 10-6 bars for calcicpyroxene (Lindstrom 1976);6.3 to 13 for hornblende(Luhr & Carmichael 1980), 12 for ilmenite (Ring-wood 1970), and 4,2 at an f(Or) of 10-13 bars to0.1I at an f(Oz) of 10-12 bars for magnetite (Lind-strom 1976). In samples 86-399 and 86-391, whole-rock V contents are 200 and 356 ppm, respectively,yet vanadium is below detection limits (<0.5 fi.qo)in microprobe analyses ofpyroxene, hornblende andilmenite. Magnetite probably is the principal V-bearing phase for these samples. Vanadium enrich-ment has been noted in other volatile-related systems,e.g., in the Kennedy's Vale ultramafic pegmatite

III

GZ

IIITPZI

I

? goo

IIl

GZ

tIJTPZ

Iv

THE MONTCALM GABBROIC COMPLEX. ONTARIO 461

1II

GZ

IIITPZ

I

qvo

\PVo

c

o

AN CONTENT Mg NUMBEF

Frc. 9. Plagioclase An contents and clinopyroxene Mgnumbers versrr.s stratigraphic height. Triangles aremicroprobe data: C core, R rim, unlabeled: defocussedspot analysis of core for plagioclase. Circles are nor-mative An contents and Mg numbers adjusted for sul-fide content. Microprobe data are in Barrie (1990).

+0.6 and -0.4 @arrie & Shirey, submitted). MGCsamples range from eNa(t) : + 3.8 from the-upperPZ,to +2.4 and +2.8 for the CZ,to + 1.0 fortieFZ. GZ cumulates have isotopic signatures similarto the depleted mantle elsewhere in the southernAbitibi Subprovince (Dupr6 et ol- 1984, Machadoet ol. l98fD: In comparison, the FZ sample is iso-topically enriched and outside the analytical error'the upper PZ sample is more depleted than thedepleted Abitibi mantle' and may have been sub-jected to REEmobility after crystallization (discussedbelow).

Bose and precious metal geochemistry

Logarithmic values of S, Ni, Cu and Au areplotted versus stratiglaphic height in Figure !Z' Jneiower PZ has relatively constant S, decreasing Ni,slightly increasing Cu and erratic Au concentrationswiin neigtrt. Above 100 m these elements increasedramati&ly, particularly Cu and Au, which reach2450 ppm and 75 ppb, respectively. The elevatedvalues are found in samples where primary cumu-late textures are partly or completely obscured bypostcumulus pegmatitic phases. Immediately above

body of the Bushveld Complex (Van Rensbure 1965)and in roscoeite-bearing (vanadium-bearing mica)hydrothermal veins in the Porgera porphyry Cu-Au deposit of Papua - New Guinea (Henry 1988).

Incompatible and immobile elements such as Ti,P , Zr, Y and La are highest in the lower PZ' thendecrease through the upper PZ to the PZ-GZ con-tact, and with few exceptions remain low throughoutlhe GZ. High values in the lower PZ are consistentwith a higher proportion of trapped liquid near themargin of the complex. Variations in La and Sm inthe GZ tend to parallel Al2O, content, reflectingtheir slightly higher partition coefficients intoplagioclase than Yb, Zr and Y. At 345 m, moreclosely spaced samples show a decrease in concen-tration of incompatible trace-elements, accompaniedby a decrease in Al2O3, and increasing MgO andFe2O3. These trends may reflect modal layering,with increasing clinopyroxene and decreasing inter-cumulus phases in the cumulates. However, thepresence of cumulus olivine at 308 m suggests thatmagma replenishment occurred during GZ forma-tion, which could also account for trace-element var-iations in the GZ stratigraPhY.

Ratios of incompatible trace-element concentra-tions can be useful in tracking the composition ofthe interstitial liquid. In general, Zr/Y and chondrite-normalized LalSm and LalYb all increase from theupper PZ to the middle GZ, and then slightlydecrease. La and Sm are slightly more compatiblein plagioclase than Yb, Y or Zr, so that La,/Sm andLa/Yb may reflect, to a certain extent, the propor-tion of cumulus plagioclase in addition to the com-position of the interstitial liquid. All ratios suggestthat the interstitial liquid becomes relatively enrichedin La and Zr in the middle GZ.

Neodymium isotope geochemistry

Neodymium isotope signatures have been deter-mined by Barrie & Shirey (submitted) for four MGCsamples as part of a regional isotopic study on theevolution of the Kamiskotia - Montcalm area. Theresults of the analyses are reported as e values cor-rected to the Montcalm U-Pb age, or the differencefrom a chondritic (CHUR) reservoir in parts per tenthousand a|2702 Ma. The 2-o errors average + 0.6e units. In general, positive eNa(t) values reflect along-term, lieht REE depletion in comparison toCHUR (e.g.,the MORB mantle reservoir), whereasnegative values reflect long-term light REE enrich-ment (e.g., old crustal rocks). Mantle-derived,uncontaminated Abitibi Subprovince rocks, such asthe Alexo komatiites, have depleted e1a(t) valuesaveraging *2.7 (corrected after Dupr6 el al.1984),whereas the isotopically most enriched Abitibi rocksare found l0 and 20 km to the east of the MGC,where two granitic samples have ero(t) values of

500

wYoov

"gP9 V

\ P V

C R!w

o

c o F_wv

q qF&w 9 w

o

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200

E

Igul-

v v

$:t:a;.:a: : : : : : , : i l i { t :

lFrFo

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800 -t-100

462 THE CANADIAN MINERALOGIST

II

GZ

II

-!_

IPZ

I

E

FT

IuJE

1II

GZ

IIITPZ

I

70 75 80W}()LE ROCKMg Number

cort,

a

a- a a

aa

aa

a

a

0.4 0.8 0.05 0.1 400 1200 10 20 30 200 400

TlO2 P2OS Cr Sc V

FIc. 10. Whole-rock major and transition-element contents yersrs srratigraphicheight. a. Major elements (wt.r/0, and Mg number, corrected for sulfide (see Table2). b. Minor (wt.q0), and compatible trace-elements (ppm).

40 50

st()2

6 0 ' t 0 2 0

Mso10 20

Fe2O3colr,

10 20

At203

a

aa

a

arrt r

aa

aa

a

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FIoutE 200

I

thePZ, S contents are very low in the lower GZ arrdthen increase sporadically but consistently to 430 m.This trend is paralleled in a general sense by Ni, Cu,and Au. Metal contenlr nelglalized to S show spu-rious enrichments at 200 m and above 430 m due tolow S contents at these levels. Anomalously low Scontents may be the result of sulfide resorption intothe parental magma shortly after formation (vonGruenewaldt e/ a/. 1986) or into a sulfur-undersaturated interstitial liquid; alternatively, theymay be due to subsolidus alteration.

Twelve of the samples in the traverse were selectedfor platinum-group element (PGE) analysis by radio-chemical neutron activation (RNAA) using themethod described in Briigmann el a/. (1987). Thismethod has very low detection limits (Os: 1 ppb; Ir:0.02 ppb; Ru: 2 ppb; Pt: I ppb; Au: 0.02 ppb) andis ideally suited for the Montcalm suite. Samples wereselected to reflect contents of primary magmatic sul-fide, with the exception of sample 9, taken as anexample of the upper PZ rocks that have undergonepostcumulus re-equilibration with a fractionated

tL t

aa

THE MONTCALM GABBROIC COMPLEX, ONTARIO 463

interstitial liquid. All PZ samples regardless of tex-ture are relatively enriched in total PGE in compar-ison to the GZt however, Au is on average equallyabundant in the two zones (Fig. 13). The PGE arebelow detection limits in all but one of GZ samples,in spite of the high sensitivity of the RNAA tech-nique. The exception is sample 86-429, which hasa low sulfur content but ahieh Mg number and highCr content in comparison to other GZ samples.

THe MoNrceLM Ni-Cu Dnpostr

Geology

The Montcalm Ni-Cu deposit is located at theextreme northern end of the MGC (Figs. 2-4).lt. iscomposed of two subvertical sulfide-rich lenses,termed the West Zone and the East Zone, each witha maximum subsurface strike-length of 200 m, anda maximum width of 25 m (Fig. l4). The West Zonecontains 1.75 million tonnes of l.5l wt.9o Ni, and0.74 wt.tlo Cu; it is almost equal in size and gradeto the East Zone. which has l.8l million tonnes of1.39 wt.9o Ni and 0.61 wt.9o Cu. No systematiczonation ofNi or Cu grade is present, except for pos-sible local enrichments near the granodiorite dike,where massive sulfide generally is more highlydeformed. The zones are hosted by medium- tocoarse-grained gabbros of the GZ and are separatedby a subvertical granodiorite dike. The wallrockgabbro contains discrete high-strain zones charac-terized by a well-developed flattening fabric, alongwith an elongation fabric locally.

Three texturally distinct types of sulfide mineralization are present. In order of decreasing abundance,these are: disseminated sulfide (10-50V0 sulfide byvolume), massive and net-textured sulfide (>50V0sulfide) and inclusion-breccia sulfide (>5090 sul-fide). The first two groups vary from a relativelyundeformed state, with easily discernible primaryigneous textures to highly foliated, locally lineatedmaterial. Foliation planes are oriented at 50o to thedrill-core axis and probably represent subverticalnorth-trending attitudes. Disseminated sulfide gener-ally is interstitial to altered silicate phases. In moredeformed and altered rock, fine-grained sulfidephases are found within tremolite-actinolite andchlorite, which are pseudomorphous afterclinopyroxene and plagioclase, respectively. Inundeformed net-textured sulfide, most or all of theinterstitial space is occupied by sulfide. Inclusion-breccia sulfide contains angular and rounded frag-ments of gabbroic, pyroxenitic and siliceous materialup to 20 cm in maximum dimension. The majorityof the fragments appear to be locally derived fromgabbroic wallrock. However, the presence ofpyrox-enitic fragments (10q0 of the fragments) in theinclusion-breccia mineralization suggests that at least

some of the fragments were derived from an ultra-mafic part of the intrusion, or from a magmachamber in the lower crust.

Mineralogy

The Ni-Cu deposit has a relatively simple miner-alogy of pyrrhotite, pentlandite and chalcopyrite'with minor magnetite, pyrite and violarite. Inundeformed net-textured sulfide, pentlandite exso-lution "flames" are preserved within pyrrhotite.Elsewhere, pentlandite occurs as euhedra sur-rounding pyrrhotite grains. Chalcopyrite is com-monly observed as millimeter-wide veinlets thatlocally cut both silicate and sulfide grains, as pres-sure shadows to fragments in the inclusion-brecciasulfide, and as fine-grained disseminations within thealtered remnants of silicate phases. Magnetite is com-monly located at sulfide-silicate contacts, and maybe the product of desulfurization reactions betweenpyrrhotite and silicate phases. The presence of pyritemay represent the local sulfurization of pyrrhotite.Violarite is found along pentlandite grain marginsin highly deformed ore.

Geochemistry

Base- and precious-metal concentrations are givenalong with S, Se, and 63aS values for 21 samplesfrom the sulfide lenses in Table 3; interelement andcorrelation coefficients with distance to the nearestfelsic dike are given in Table 4. These samplesrepresent the range of compositions and texturesfrom the West and East Zones. Average metal con-tents in 10090 sulfide derived from these data havebeen presented previously (Naldrett & Duke 1980).Drill-hole locations and intervals are given in Barrie(leeo).

There are few significant interelement correlations(Table 4). Sulfur correlates with Ni and Co asexpected, but does not correlate with Cu. This lackof correlation may be due to the rheology contrastbetween chalcopyrite and Fe-Ni-Co sulfides. Themore malleable chalcopyrite tends to flow understress and separate from other sulfides (Kelly & Clark1975). Rhodium correlates with S and Ni, and showsa weak negative correlation with Cu, implying thatit may be incorporated into the structure of pent-landite but not that of chalcopyrite. None of the ele-ments correlate with distance to the nearest felsicdike, suggesting that element mobilization due todike emplacement was either random or not signifi-cant for this PoPulation.

S/Se ratios and 0345 values have been used asindicators of sedimentary sulfide contamination inmagmatic deposits. Ideally, a primitive magmashould have a chondritic S/Se value of approximately2700 (Aller 1967); values of2000 to 10'000 are con-

464 THE CANADIAN MINERALOGIST

?..rr!

aa

a

1 0

Y

ii#ii;i,l:?;::*.

i::;;*:,:::*::,*.

sidered lormal for uncontaminated magmatic sul-fide ores (Naldrett l98l). Archean sedimentary sul-fide has a variable ratio depending on the reductionstate of the depositional environment, but it is gener-ally in the range of 20,0m - >50,000 (Shegelski1978, Green 1978). S/Se values for sulfide-oxide-facies iron formation in the Kamiskotia area average38,500 (n = 12: Barrie 1990), and for Langmuirtownship 85 km to the ESE, they average 23,700 (n: 1l: Green 1978). For Montcalm, the S,/Se value

a.

a-4.

!a

a

a

a

a

a

" , .aa

r r oa

a o a

E

F 300Igutr 2oo

I

II

GZ

II,tI

PZI

I

III

GZ

IIJTI

PZ

I

a

t i : ::a .I rqi

a

aa

a

aaa

a

a

a aa

a

aa

a|b

J

aa

aa

a

a aa

500

400

aa

]t.'j|',4iiiiiii:

E: 3ooIoutr 200

a

a- r o

a

a

L

is 13,000 (n : 20, lo error : 8500), nearly withinthe range of normal magmatic sulfide ores.

Ideally magmatic sulfides derived from primitivesources will have chondritic 6315 values of 07oo (withrespect to Canyon Diablo troilite). Biogenic fraction-ation of sulfur isotopes generally causes an increasein 34S with respect to 32S in sulfate and sulfideminerals, and consequently in sedimentary host-rocks. Although there is ample evidence for biogenicactivity during the formation of the southern

2 4 6 7 2 0 . 5 1 1 . 5 1 2 3 2 4 6 8

La Sm Yb LalSm)cn La/Yb)cn

Ftc. I L Incompatible trace elements versas stratigraphic height. a. Zr, Y (ppm) andZr/Y. b. RAE'@pm) and REE ratios.

THE MONTCALM GABBROIC COMPLEX. ONTARIO 465

a

a III

GZ

IIJ

T

IPZ

Iaa

- a a

aa

aa

a

a- a a

aaa

a

a

3

log S

Superior Province (Wilkes & Nisbet 1985), the evi-dence from sulfur isotopes for this activity is ambig-uous: sulfides from the Michipicoten and WomanRiver iron formations in the Wawa Subprovince have63aS values that range from -10 to + 10/oo(Goodwin et al. 1976). The 6345 values for theMontcalm deposit average -1.80/oo with little devia-tion (n = 10, lo : 1.2), consistent with a primitivemagmatic source for sulfur. It is noted that this doesnot rule out a sedimentary source for the sulfur.

The Montcalm deposit has average base-metalcontents normalized to 10090 sulfide of 4.31 wt.YoNi, l.38Vo Cu and 0.?.4t/o Co (Naldrett & Duke1980). In terms of its base-metal content, it is mostsimilar to typical Sudbury ores, which have3-7 wt.t/oNi, 2-690 Cu and 0.1590 Co in 100V0 sulfide. Flood-basalt-related deposits such as Noril'sk, and Min-namrx and Great Lakes Nickel of the Duluth dis-trict, have similar sulfide-normalized Ni and Co con-tents of 3-7 and 0.15-0.20 fi.V0, respectively, butthey have much higher Cu contents (8-17 wt.qo: Nal-drett l98l).

In terms of the PGE and Au, the Montcalmdeposit contains conspicuously low concentrations,nearly two orders of magnitude less than flood-basalt-related deposits (Fig. l5). There appears tobe a negative Pd anomaly, and the normalized Aucontent is relatively enriched with respectto the PGE.The steep pattern, with much lower abundances ofOs, Ir and Ru, is typical of gabbro-hosted Ni-Cudeposits. This is in comparison to Ni deposits foundin komatiites, which have flatter and more chondritic

aa

i: ;iir:ir:,:.:::::::.::*:*.

f . 5 3 1 2 3 0 I

tog Nl log Cu log Au

E- 300FIglllr 200 a a

a

."#:;::il:::Y"'r: : i l r i : r ,a

aa

a

Frc. 12. Log ppm S, Ni, Cu and Au ver.tt s stratigraphic height.

GZ

IIItPZ

II

EFIC'trr

20 40 60ppb

Au-PGE

Frc. 13. Total platinum-group-element (PGf) and Aufupb) concentrations versas stratigraphic height. Pal-ladium included in total PGE; estimated as Pd*.01. =Ptwple * @d6"oor1/Pt4.oor1).

466 THE CANADIAN MINERALOGIST

OVERBURDEN

Frc. 14. Ni and Cu values (wt.9o) for Ni-Cu depositlenses. Below: location of cross-section. Modified afterTeck Corporation files.

patterns. Other gabbro-hosted Ni-Cu depositshaving low PGE tenors and similar PGE patterns arethe deposits at Moxie (Thompson 1982, Thompson& Naldrett 1984), the Bruvann deposit (Boyd et al.1987) of the Rdna Intrusion, Norway, and severaldeposits in Svecokarelian migmatite terrain ofsouthern Finland (Papunen 1989).

DIScUSSIoN

Composition of the parentol magma: evidence fromthe lower Pyroxenite Zone

Although the lower PZ, the upper PZ and the GZhave been affected by different cumulus and post-cumulus processes, they all exhibit light REE enrich-ment. which is believed to be a characteristic of theoverall parental magma composition @ig. 16). LowerPZ rocks most closely approximate a liquid compo-sition, owing to their orthocumulate texture and theirrelatively high contents of incompatible elements.The REEpatterns for lower PZ rocks (Fig. l6a) havechondrite-normalized La in the range of 2l-25,La/Yb in the range of 2.4-3.8, and no significantEu anomaly. On average, they show a slight deple-tion in Ta-Nb-Ti and Zr-Hf if normalized to MORBor primitive mantle values.

There are several ways for elevated R.EE'abun-dances with mild LREE enrichment to occur. Onepossibility is partial melting of the mantle with asmall proportion of garnet in the residuum, followedby olivine fractionation to increase concentrationsof the incompatible elements. This proposal is notviable, as the high degree of partial melting requiredwould consume all of the garnet in reasonable garnetperidotite compositions, and no ZREE enrichmentwould occur. A second possibility is that metaso-matic activity in the upper mantle may have addeda LREE-eniched, volatile or siliceous component tothe upper mantle prior to partial melting and extrac-tion of the PZ parental liquid. This process has beenenvisioned f or L I LE and ZREZ enrichment, accom-panied by a relative high-field-strength element"depletion", in modern continental arc settingsduring the generation of alkaline basalts (e.9., Pearce1983). Selective contamination is a third possibility,with a primitive magma gainingthe LREE preferen-tially from crustal rocks during ascent through thecnrst, again followed by olivine fractionation. Selec-tive contamination is believed to be a viable methodof extraction of alkalis and silica from crustal rocks(Watson 1982), but it has yet to be demonstrated forthe REE, A fourth mechanism is bulk assimilationof continental crust by a primitive ultramafic magmaduring olivine fractionation at lower crustal levels.Using the average Archean crust of Taylor &Mclennan (1985) as a contaminant, the REE abun-dances for lower PZ samples can be derived froma35t/o partial melt of the primitive mantle, followedby 200/o olivine fractionation accompanied by 20a/oassimilation. However, Zr, Hf and Th are tooenriched in this model. Of these alternatives, themost viable explanation for LREE enrichment in thelower PZ would appear to involve metasomatism inthe mantle prior to partial melting and extraction ofPZ parental liquids.

T ++ -+

lI

\ ilv

+

>i.

o#tr{',,rrril

$un

THE MONTCALM GABBROIC COMPLEX. ONTARIO

lslt 3. G!@HiltsrRY or sslllt sulf l0[ wtEs lm ilt &lrdu ti'c! Dfmslr

467

s

8T

6 3 l s

1 . 9 E 2 6 0 L 3 1 3 6 3 5 8 2 . 4 9 2 0 9 2 . 1 5 2 . 7 2 2 . 6 t L l ! 3 . E 5 ? . 6 0 1 . 7 7 r . 3 0 3 . 7 3 1 . 8 2 2 . 6 9 t . 5 r l ! 4 1 . 7 0

0 . { 3 I 0 E 0 2 1 0 2 a 0 5 l r 0 6 1 a r l . ! l 0 . 6 6 r . 2 0 0 . 9 8 0 . 2 0 0 . a a 0 . 5 2 0 . { ! 0 . 5 9 1 . 8 8 0 . 5 0 l . 1 5 1 . 3 3 0 . 2 2

0 0 E 0 1 5 0 0 7 0 . 0 5 0 2 1 0 l t 0 . 1 6 0 t 0 0 . t a 0 . 1 8 0 1 9 0 2 t 0 . 2 1 0 . 0 6 0 . ? 0 o . l l 0 . ! t 0 . t l 0 . 1 ? 0 . 3 0 0 . r 9

1 7 . 9 2 2 1 1 3 . 6 t 9 . s 3 { g 3 0 5 l 3 . E 2 ! . 9 2 2 6 ? E . 0 3 0 . 6 3 3 . 6 2 7 . 3 1 1 . 8 3 2 . 5 2 5 . 1 1 6 5 2 t 0 2 1 0 2 6 . 5 1 0 . 5

! r 2 . 6 E

C u 0 . 8

co 0 . 15

s 2 4 . 1

0 .69 Z l l . ( )1 ,00

0. 5 2 l 0 .68 1500

0. 06 2 l

6 . 5 2 l

2? 22 < t3 35 ?O

{0 56 2 t E l 2425 24 \6 22 15

3 0 3 1 9 3 1 1 9

! 9 1 2 0 8

1 . 5 1 . 6 1 . 6 I 8 2 . 6

l . t l 0 2 (0 . 5 0 r 0 . t o 7 0 . 1

39 r0 l 26 9 t6

' 4 1

?02 l

2 l

2 \2 1

l 9

2 l

? l

z l

l 0

20

8!i !

63 ls

5 /S i

19 ?8 3r l2 18 16 . . 25 31 23 . r l

13 ! 66 r l 35 l l 3E t6 32 21 6 t 26

22 2 t t2 2 t 1 l l7 22 19 l l 2 l t !

2 1

59

104

t 2l . 6

3

! . 4

3 l

l 2

l 5

I

50 5! 6a 31 5o 22 22 321 8 1 5 1 2 i 3 3 5 9 3 3

l . I l . 2 < l ( 1 3 . 7 . . . O 2 . 6f 4 5 r ? < t ( 1 2

0 . 3 0 . 6 0 . 4 0 l 1 . 2 0 . 3 0 . { 0 . 352 6a 8 ! 27 18 19 39 ?E

29 Z l

t . 3 l . a

2 3

0 . 5 0 . 4

3 3 3 l

15 3 ! 40 aEq 8 < l l 9

d ( l l . 0 4 , 0

{ 2 < l l

0 . 2 0 . 2 0 . 1 0 . 4

l0 l l0 E{ 19

0 . 8 - 0 . 7 . . . - 1 . 2

pa?3 18 3r l.a5 15 l l lal l l1 t3 P!

2 3 . 5 9 . 8

50 27\ 1 . 1 t . 8

35.6 20 |

9 . I 8 , {

o . 7 0 . I< 3 . 5 6 I

0 . 1 0 . 2 1

15 26 . 7

. 1 . 8 t . 2

1303t E59r

0 . 1

4 6

l 8 . 0 . 3 . 2 . ? . 1 . 3 , ? . 0 . . . - 3 . 1 . 1 . !

s/sr El36 10015 1016? !571 17100 t357r E815 1t000 8071 8?35 2t500 7000 17053 r3000 718? T l t r 19091 9130 11722 E97 l

l f tns i l lon hr t ! l ! rn r t r r id ! , & f ud M ! t r . { ! t A3s ! t L .bor . t0 r t r ! , ro ron !o i psL and Au .n r t t l rd ! t m r l t ! t l !o ld 9 r .co !c !0 t ra t lon i t ! ! io l lo t ! ! 3 l ! l (197E) to t da t t l l s 0n

TABLE 4. COREI-ATION C0EFFICIEMS (r) FOR Ni-Cu DEmsIT SNPLES=

0 . 0 8- o . 4 6

- 0 . 0 8- 0 . 2 8

N IC o 0 . 4 8C u - 0 . 3 6

P ! 0 , 0 7P d 0 . 3 8R h 0 . 1 6R u - 0 . 3 5

I r 0 . 0 9A u - 0 . 1 6

s 0 . 7 3

0 . 0 60 . 2 3 0 . 3 00 . 0 7 0 . 0 5 0 . 5 30 . 3 5 - 0 . 5 7 0 . 0 4

- 0 . 3 8 - 0 . 2 3 - 0 . 3 4

0 . 0 9 - 0 . 3 s 0 . 3 00 . 1 4 0 . 3 2 0 . 0 10.82 -O-_14 0. 19

- o . 2 r

0 . 3 8 - 0 . l 8- 0 . 4 6 - 0 . 0 3 - 0 . 2 4

o . 6 a - 0 . 4 8 0 . 0 6 - 0 . 0 4

o . t 40 . 0 60 . 1 0

- 0 . 3 6-o-02- 0 . 3 0

o . 2 3- 0 . 2 8

0 . 0 60 . 1 2

Dia tance (E)

* S a n p l e s l i s t e d i n T a b l e 4 .

R va lues > O.50 in bo ld l ype ; o ther no teFor thy va lues under l j .n€d

Zone refining in the upper Pyroxenite Zone

Upper PZ rocks have textures that suggest partialor complete remelting in the presence of a volatile-rich phase. These include: l) coarse-grained and peg-matitic textures with intercumulus hornblende andFe-Ti oxides after cumulus phases, 2) gabbro peg-matite gradational with pyroxenite cumulates; and3) pyroxenite and gabbro pegmatite dikes. Thepresence of a volatile-rich phase during postcumuluscooling can lower the solidus temperature of pyrox-e[ite cumulates and cause partial melting. The newlyformed melt will migrate through the cumulates,leaving behind a residue rich in compatible elements(e.9., pyroxenite dikes), while the melt becomesincreasingly enriched in incompatible elements,analogous to zone refining in the metallurgicalindustry (Harris 1957, McBirney 1987). Enrichmentin incompatible elements will occur where thevolatile-rich melt crystallizes (e.9., gabbro pegma-tite dikes). As in hydrothermal systems, repeatedpulses of volatile-rich melt may follow the samedirection and ultimately leave a complex signature.This process is similar to infiltration metasomatism

described for pegmatitic hornblendites at DukeIsland in Alaska by Irvine (1974,1987).

The origin of the volatile-rich fluids is not wellunderstood. The pegmatitic hornblendites at DukeIsland, exposed over hundreds of square meters, aremost prominent near the margins (Irvine 1987), withthe inference that the infiltrating fluids were derivedfrom surrounding rocks. At the Skaergaard Intru-sion and several of the Tertiary layered intrusionsof East Greenland, gabbro pegmatite dikes, veins,and pods are especially abundant in the MarginalBorder Series, and the volatile-rich fluids responsiblefor their formation are believed to be derived fromadjoining country-rocks (Taylor & Forester 1979,Irvine 1987, Bird et ol.1988). This fluid infiltrationwould have to take place while the cumulates werestill hot enough to be remelted under hydrous con-ditions. Geothermometers indicate that calcic amphi-bole + pyroxene assemblages in subsolidus pegma-titic veins up to 2 cm thick at Skaergaard formedat525"C to 825oC, under hydrostatic conditions at0.5 kilobars (Bird el a/. 1988). Experiments on themelting of magnesian basalt with P(H2O) =P(total) indicate that at2 and 4 kilobars, the solidus

468 THE CANADIAN MINERALOGIST

o

c

o

o

o

o

s

o

o.1

o.ot

matitic pyroxenite and gabbro pegmatite depleted inincompatible elements, and precipitating anincompatible-element-enriched residuum, partly dueof saturation of incompatible-element-enrichedphases. Given this interpretation, a V-rich phase,probably a Fe-Ti oxide, became saturated in thevolatile-rich melt before a sulfide phase that con-tained high Cu and Au contents.

In many respects, the upper PZ is similar to theRobie Zone of the Lac des Iles Pt-Pd-Au-Cu-Nideposit in northwestern Ontario (Macdonald 1987).Gabbros of the Robie Zqne with textures similar toupper PZ rocks host significant concentrations ofPt-Pd mineralization that has been attributed to thezone-refining process (Briigmann et al. 1989). Unfor-tunately, results of semiquantitative Pt-Pd-Au ana-lyses for PZ pyroxenites and gabbro pegmatites donot indicate any significant enrichment like that atLac des Iles (Barrie, unpubl. data).

HiCh Pt/Ir values, and derivotion of the metol con-tent in the Ni-Cu deposit

The high Pt,/Ir values of the lower PZ cumulates(144, n = 4) are comparable to those found in pyrox-enitic and gabbroic rocks of the Thetford ophiolite(71 and 265, respectively: Oshin & Crocket 1982) andgabbros associated with Munro Township komatiites(55: Crocket & MacRae 1986). These values are sig-nificantly higher than the chondritic ratio of 1.9 (Nal-drett & Duke 1980), or Pt/lr values found in ultra-mafic tectonites and cumulates at Thetford(0.05-11.7: Oshin & Crocket 1982) or at MunroTownship (7.8: Crocket & MacRae 1986). Further-more. the Ptllr value of the Montcalm Ni-Cudeposit (90, n : 20) is similar to values for the Min-n€rmax and Great Lakes Nickel deposits of theDuluth Gabbroic Complex (55-75: Naldrett l98l),but much higher than for komatiite-hosted deposits(3.3-14.4: Naldrett & Duke 1980). Partial meltingand fractional crystallization are considered viablemechanisms to produce high Pt,/Ir values (or Pd/Irvalues, as Pd and Pt behave similarly) in primitivemagmas and derivative Ni-Cu deposits (Barnes e/al. 1986, Naldrett & Barnes 1986). In both cases, thearguments are based upon the assumptions that Irbehaves compatibly in olivine and chromite, whereasPt and Pd are incompatible. Bri.igmann et al. (1987)determined an olivine - komatiitic liquid partitioncoefficient for Ir of about 2, and there is petrolog-ical evidence that chromite fractionation removes Irby nucleating around laurite and Os-Ir-Ru alloys(e.9., Stockman & Hlava 1984).

It is difficult to envision partial melting from aprimitive mantle as the principal mechanism toincrease the Ptllr value at Montcalm, for tworeasons. First, clinopyroxene and whole-rock Mgnumbers up to 82 and HREE contents in the lower

o.oo1

Ftc. 15. PGE and Au average abundances for MontcalmNi-Cu deposit in comparison to typical komatiite-hosteddeposits (Langmuir, Mount Edward, Kambalda), andorher gabbro-hosted deposits. Modified after Naldrett& Duke (1980) and Naldrett (1981).

temperatures are 700 and 825'C (Holloway &Burnham 1972). These temperatures would representminimum temperatures for the fluids responsible fordevelopment of the upper PZ pegmatitic rocks.Interestingly, this is within the upper limits of fluidsthat have evolved from granitic plutons (Burnham1979, Beane &Titley l98l). Ifthe contemporaneousfelsic dikes that cut the GZ and PZ represent theupper reaches of a larger felsic pluton at depth, thenthe fluids could be derived at least in part from sucha pluton.

In the upper PZ, the REE are lowest near the GZand progress to higher levels down-section; this isparticularly prominent for S, Cu and Au. Thesetrends would suggest that a volatile-rich fluid origi-nated near thePZ-GZ contact, possibly from a con-duit near one of the contemporaneous felsic dikes.The migrating fluid may have caused partial meltingof pyroxenite cumulates, leaving a residuum of peg-

t t t t t t lOs lr Ru Rh pt pd Au

THE MONTCALM GABBROIC COMPLEX, ONTARIO

lower GZ

La Ce NdSmEu Tb YbLuFIo, 16. REE diagrams, normalized to average CI chondritic values of Evensen el

al. (1978). a. Pyroxenite Zone, with lower PZ (vertically ruled) and upper PZ(horizontally ruled). b. Gabbro Zone, with lower GZ (<300 m: vertically ruled)and upper GZ (above 300 m: horizontally ruled).

469

2

13020

10

PZ suggest approximately 20-30t/o partial meltingof a primitive mantle composition. At these amountsof partial melting, it is unlikely that sulfide wouldremain in the mantle as a restite phase, so that thePt. Pd and Ir content of the melt would be controlledby their partition coefficients with mafic silicatephases. Naldrett & Barnes (1986) modeled20 - 30s/opartial melting of a sulfide-free primitive mantlecomposition, and found Pdllr values of 10-20, com-parable to Pt,/Ir values of 5-10, given a chondriticratio of about 2 for PtlPd in the mantle. The parti-tion coefficients between sulfide liquid and silicatemagma are not well known, but are believed to bevery high (> ltr) and similar to one another, so thatsulfide derived from these magmas would havesimilar Pt,/Ir values of 5-10. These values are muchlower than those of the lower PZ cumulates and theMontcalm Ni-Cu deposit. Second, the low PGEtenor of the deposit cannot be reconciled with thebase-metal tenor after segregation of an immisciblesulfide liquid from a primitive picritic magma, forany ratio of silicate magma to sulfide liquid, as

oL

!tco.c,oUIlrJTT

3020

10

543

5432

pointed out by Naldrett & Duke (1980). Whereas alarge batch of sulfide liquid in equilibrium with amagnesian tholeiitic melt (a low "R" factor: Camp-bell & Naldrett 1979) could account for the low PGEtenor. the Ni and Cu contents would be too low. Par-tial melting from a mantle source that had under-gone a previous event of partial melting that leftbehind trace amounts of sulfide, as suggested forboninitic magmas (Hamlyn et al. 1985), could alsolead to a higher Ptllr value; however, this scenariois not considered applicable to Montcalm in view ofits tholeiitic affinities.

Fractional crystallization with continual segrega-tion of sulfide from a primitive magma, as originallyproposed by Naldrett & Duke (1980), is a reasonablealternative. They modeled the sulfide that would bein equilibrium with a magnesian tholeiitic liquid afterfractional removal of 50Vo olivine, clinopyroxene,plagioclase, and continuous removal of traceamounts of sulfide. By this method, they achieveda sulfide liquid with 4.0 wt.9o Ni, l.3Vo Cu and 17ppb Pd in l00Vo sulfide, very similar to sulfide-

470 THE CANADIAN MINERALOGIST

60pt (ppb)ao

o

o

o - n

o

. o;'

-. n- -Eo.,r'

(ppm)

Relative timing of the immiscible sulfide event duringthe evolution of the MGC

The timing of the immiscible sulfide event that ledto the formation of the Ni-Cu deposit is constrainedby the following considerations: the presence ofsul-fides in thePZ, but very low S and PGE concentra-tions in the GZ, implying sulfur undersaturationduring GZ formation; similar, but slightly higher,more fractionatedPt/b values in thePZ than in theNi-Cu deposit; 6345 and S/Se values consistent witha magmatic derivation of sulfide; and depleted iso-topic signatures for the P Z and GZ , but an enrichedsignature for the FZ. These considerations are con-sistent with the following sequence of events: l)ponding of a primitive partial melt from the mantlein the lower crust, and fractionation of olivine, chro-mite and trace amounts of sulfide; 2) migration ofthe residual liquid from the lower crustal chamberto the MGC chamber, to form pyroxenitesrepresented by thePZ and the pyroxenite fragmentsin the inclusion-breccia sulfide; 3) formation of themain immiscible sulfide liquid from the PZ parentalmagma, either in a lower crust chamber or the MGCchamber; 4) continued fractionation into the stabilityfield of plagioclase, accompanied by influxes of newmagma, and the formation of GZ cumulates at2702+ 2Ma;5) assimilation of an isotopically enriched

I

6 t r

4

2

(ppb)

20

100

Pt/lr 10

1500s

1000

5001

0.1

201 510 25 1 510 2520Ni/Co

FIc. 17. Montcalm metal contents in comparison to Thetford Ophiolite Complex data (Oshin & Crocket 1982), Mont-calmPZ rocks: open squares, Montcalm GZ rocks: open circles, Thetford lherzolites, dunites, and pyroxenites: closedcircles, Thetford gabbroic rocks: closed squares, a. Ni/Co versusPI. b. Ni/Co versusb. c. Ni/Co versusPt/b.Star represents Montcalm deposit average. d. Ni/Co yersas S.

normalized values calculated for the Montcalmdeposit. This model also accounts for higher Ptllrvalues, as olivine (and chromite) fractionation woulddeplete the magma in Ir.

A schematic illustration of this model, involvingsilicate fractionation accompanied by sulfide segre-gation from a primitive magma, is given in Figure17, where Montcalm data are plotted along with datafrom from the Thetford Ophiolite, Quebec (Oshin& Crocket 1982). The Thetford suite includes uppermantle harzburgite and peridotite tectonites, in faultcontact with a cumulate sequence of partly layeredcumulates. The cumulates are chromite-bearingdunites at the base, overlain by pyroxenites andgabbros. Ophiolite suites including Thetford aregenerally regarded as the product of open-systemfractional crystallization (e. g., O'Hara I 977, Pallister& Knight 1981). Ni/Co is used as a general index offractionation for these cumulate rocks. Ni is favoredover Co by 2-4 times in mafic silicates and spinel(see Irving 1978), and 3-5 times in sulfide (Rajamani& Naldrett 1978), so that Ni,/Co decreases systemat-ically with fractionation. The Montcalm traversesamples are similar to Thetford pyroxenitic and gab-broic rocks in their Pt, Ir and S contents (except forthe high S content in upper PZ rocks), and their highPt,/Ir values; the Ni-Cu deposit has a similar Pt,/Irvalue (Fig. 17c).

THE MONTCALM GABBROIC COMPLEX, ONTARIO 471

qustal component during FZ formatioa; 6) emplace-ment of the sulfide mass into the GZ after consoli-dation of GZ cumulates, during tectonism andregional crustal rotation into a subvertical position;7) emplacement of the main granodiorite dike at2700105Ma and subsidiary felsic dikes; 8) volatilefluxing causing remelting and recrystallization ofupper PZ rocks; and 9) deformation associated withthe emplacement of granitic plutons to the east at2696-2692 Ma.

CoNcI-ustoNs

The principal conclusions drawn from the geology,geochronology, petrology, and geochemistry of theMGC and Montcalm Ni-Cu deposit are summarizedas follows:

l. The MGC was emplaced at2702 t 2 Ma in anactive tectonic setting, and was subjected to rotationof the crust into a near-vertical position during orshortly after consolidation. The MGC and associatedNi-Cu deposit were cut by a subvertical granodioritedike at 2700!n5Ma.

2. In the vicinily of the Ni-Cu deposit, the lowerPZ orthocumulates provide the best estimate of aliquid composition. They have relatively high con-centrations of the incompatible trace-elements, areLREE-enriched, and have Ptllr values similar to butslightly higher than the Ni-Cu deposit. The majorityof upper PZ rocks have coarse-grained and pegma-titic textures, and have been extensively modified bypostcumulus recrystallization due to a flux ofvolatile-rich fluids, possibly derived from contem-poraneous felsic magmatism. GZ rocks arepredominantly plagioclase - clinopyroxene cumulateswith low levels of incompatible trace-elements andsulfur. There is limited evidence for multiple pulsesof replenishing magma during GZ formation.

3. The deposit has Ni, Cu and Ptllr values typicalfor gabbro-hosted deposits, and yet a very low PGEtenor. Values of 63aS and S/Se are consistent witha predominantly magmatic source for sulfur. Thehigh Ptllr value and the low tenor of the PGE areconsistent with sulfide segregation from a primitivemagma after fractionation of olivine, clinopyroxeneand probably chromite, along with trace amountsof sulfide. This fractionation occurred during orbefore PZ formation, as suggested by high Ptllrvalues for PZ rocks, in the Montcalm magmachamber or in a chamber deeper in the crust.

4. The parental magma was derived from an isotop-ically depleted mantle, typical of tholeiitic andkomatiitic magma sources in the southern Abitibiand similar to the depleted MORB mantle at 2700Ma. Assimilation of an enriched crustal componentoccurred during FZ crystallization.

ACKNOWLEDGEMENTS

We thank Dr. M. Blecha and Teck Corporationfor access to the drill core of the Montcalm depositand to unpublished geochemical and aeromagneticdata; Dr. A. H. Green, J. Cecchetto and Falcon-bridge Ltd. for logistical support; Dr. T.E. Kroghfor use of the analytical facilities at the Jack Sat-terley Laboratory of the Royal Ontario Museum; S.Kamo for help with U-Pb chemistry; Dr. S. B. Shireyfor help with Nd isotope data collection; Drs. G. E.Brtgmann and M. P. Gorton for fruitful discussionsabout the geochemical data; Dr. R.F. Martin andtwo referees for their editorial and scientific reviews.This work has been supported by grants from theGeological Society of America and Sigma Xi to CTBand NSERC Grant A-4244 to AJN.

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Received July ],9, 1989, revised manuscript acceptedNovember 22, 1989.